Understanding SC 1.4.1:Use of Color (Level A)

This should not in any way discourage the use of color on a page, or even color coding if it is complemented by other visual indication.

If content is conveyed through the use of colors that differ not only in their hue, but that also have a significant difference in lightness, then this counts as an additional visual distinction, as long as the difference in relative luminance between the colors leads to a contrast ratio of 3:1 or greater. For example, a light green and a dark red differ both by color (hue) and by lightness, so they would pass if the contrast ratio is at least 3:1. Similarly, if content is distinguished by inverting an element's foreground and background colors, this would pass (again, assuming that the foreground and background colors have a sufficient contrast).

However, if content relies on the user's ability to accurately perceive or differentiate a particular color an additional visual indicator will be required regardless of the contrast ratio between those colors. For example, knowing whether an outline is green for valid or red for invalid.

This criterion does not apply to situations where color has not been used to convey information, indicate an action, prompt a response or distinguish a visual element. For instance, a hyperlink which has been styled to appear no different than neighboring static text would not fail this success criterion, as there would be no color differentiation between the actionable hyperlink text and the adjacent static text. Such lack of styling differentiation could result in poor usability for anyone looking at the interface, regardless of disability.

This criterion does not directly address the needs of users with assistive technologies. It aims to ensure that sighted users who cannot distinguish between some colors can still understand content. How information is conveyed to assistive technology users is covered separately in other criteria, including (but not limited to) 1.1.1 Non-text Content, 1.3.1 Info and Relationships, and 4.1.2 Name, Role, Value.

Conversely, even if information that is conveyed by color differences is appropriately conveyed to assistive technologies, it does not necessarily pass this criterion, as sighted users who cannot distinguish between certain color may not necessarily be using any assistive technologies. This criterion requires a visible alternative to color.

Most user agents provide users with a color-only cue that a link has been previously activated by them ("visited"). However, several technical constraints result in authors having very limited control over these color-only indications of visited links. The technical constraints are as follows:

1. User agents constrain the exposure of a link's visited state due to privacy concerns. Author queries to user agents will indicate all links have not been visited.
2. Any available information on the visited state of a link would be inaccurate since it is both user and browser-dependent. Even if an author could accurately get information on previously activated links by a certain user, the author would be constrained to the current browser's preserved history, and would be unable to determine if the user had visited the page using a different browser (or if the history was not preserved, either from cache clearing or use of private sessions).
3. Authors can only use color to modify the :visited CSS pseudoclass style. The technology constrains any non-color styling. Due to palette limitations, an author cannot use color alone to achieve 3:1 contrast between link and non-link text as well as between visited and unvisited links while also achieving 4.5:1 contrast for all link and non-link text.
4. Authors also cannot set the visited state of links. The anchor element does not include a "visited" attribute; therefore the author has no ability to alter the state through an attribute setting. As such, authors cannot achieve 1.3.1 Info and Relationships or 4.1.2 Name, Role, Value in regard to visited links.

For these reasons, setting or conveying a link's visited status is not an author responsibility. Where color alone distinguishes between visited and unvisited links, it does not result in a failure of this Success Criterion, even where the contrast between the two link colors is below 3:1. Note that authors must continue to ensure that all text links meet contrast minimums against the page background (SC 1.4.3).